1. Field of the Invention
The present invention relates to a method of correcting image data picked up from pictures photographed on photographic film, especially those photographed by lens-fitted photographic film units or compact cameras, for printing pictures with improved quality on the basis of the corrected image data.
2. Background Arts
As a photographic device for taking pictures on a photographic film, the lens-fitted photo film units have been known these days besides cameras, including single-reflection cameras and compact cameras. The lens-fitted photo film unit, hereinafter called the film unit, is widely used in the world because of its handiness: it contains a roll of unexposed photographic film in a unit body having photographic mechanisms, including a taking lens and a shutter mechanism, incorporated thereinto, to permit enjoying photography quickly on the spot as it is purchased, and the user has only to forward the film unit after photography to an agency of a photofinisher. Since another advantage of the film unit is that it is purchased at a low price, the manufacturing cost should be as low as possible, so the structure must be simple, and the components must be inexpensive.
For this reason, the taking lens mounted to the above described film unit is usually composed of a single or a couple of plastic lenses. Since the taking lens is composed of a single or a couple of lenses, it is difficult to improve the performances of the taking lens so as to correct aberrations sufficiently for obtaining adequate image quality. For this reason, in the conventional film unit, in order to compensate for the curvature of field of the taking lens, one of the factors that worsens the image quality, the photographic film is held behind an exposure aperture of the film unit, with its surface curved along its lengthwise direction, so that an optical image formed through the taking lens on the curved film surface may be in focus in the entire exposure frame.
As another typical factor of worsening the image quality, distortion of the taking lens may be cited. The distortion is resulted from changes in image magnification with the distance between the exit pupil of the taking lens to the image surface that is equal to the film surface to be exposed.
The distortion and other aberrations can be well eliminated by using an accurate taking lens that is composed of a certain number of expensive lens elements, like those used in single-lens reflex cameras. However, such an expensive taking lens is not applicable to the lens-fitted photo film units and the compact cameras since they need to be manufactured at a low cost. Thus, the pictures photographed by the film unit or the compact camera cannot avoid being affected by the lens distortion.
To solve this problem, teaching to pick up image data from the photographed pictures and correct the image data so as to eliminate the influence of the aberrations of the taking lens has been disclosed for example in JPA Nos. 11-313214 and 2000-125174. Based on the corrected image data, a high quality image may be reproduced.
Specifically, according to the image processing method disclosed in JPA 11-313214, the original image recorded on photographic film or the like is optically read out through a scanner to pick up original image data, and the original image data is processed to correct the distortion of the original image in accordance with lens data of the taking lens used for photographing the original image. In the image processing, coordinate values assigned to the original image data are transformed by use of correction formulas and correction coefficients for the correction formulas which are predetermined in accordance with the lens data, and the image data is rearranged according to the transformed coordinates. In JPA 2000-125174, the image processing for correcting the image distortion is executed by use of a correction formula that represents an index of distortion f(L)=(L′−L)/L′, wherein L represents a distance from the optical axis, and L′ represents a distance varied from the distance L due to the distortion.
However, JPA 11-313214 does not concretely disclose the correction formula for the image processing, but just discloses that the correction formula is predetermined according to the lens data. If the correction formula is complicated, it will take enormous time for the calculation or operation. Thus, the image processing method using such a complicated correction formula is not applicable to a digital printer for the film units that is required to process and print a large number of images at a high speed.
The image processing method disclosed in JPA 2000-125174 merely compensates for the lens distortion. This method cannot correct the image distortion resulted from the curvature of the film surface at the exposure in the lens-fitted photo film unit or the like.